1. Field of the Invention
The present invention relates to a method and a device for developing electrostatic latent images.
2. Discussion of Background
The device for developing electrostatic latent images incorporated in the electrophotographic printers and copying machines has been investigated from various angles in order to improve the printing speed and image reproducibility. In particular, there is an increasing demand for the reproducibility of an image constituted of picture elements with high density to achieve high-speed printing and obtain high resolving power and resolution of the printed image.
In a development method using a two-component developer made of a magnetic carrier and a powdered toner, when the transporting velocity (Vp) of the surface of a latent-image-bearing member is increased to improve the printing speed, the period of time during which the developer passes the latent image formed on the latent-image-bearing member is necessarily curtailed, thereby producing many problems. For example, insufficient amount of developer lowers the image density, halftone images lack uniformity in image density, thin line images become broken, and the toner fails to transfer to a small-size dot image. In order to solve the above-mentioned problems, many trials have been made to increase the amount of developer that can be brought into contact with the latent images and to extend the period of time during which the developer is in contact with the latent images. For instance, it is proposed to increase the width (L) of a contact portion between the developer and the latent-image-bearing member, which will be hereinafter referred to as a development nip width or simply a nip width, and to increase the transporting velocity (Vr) of the developer-bearing member with respect to the transporting velocity (Vp) of the latent-image-bearing member. The above-mentioned width (L) is the width of a contact portion of the latent-image-bearing member with the developer in a direction of the transporting direction of the latent-image-bearing member.
However, it is known that extension of the contact time between the developer and the latent image at the nip width, which will be hereinafter referred to as a nip time, and increase in amount of the developer which comes in contact with the latent image bring about abnormal images. To be more specific about the abnormal images, the image density of a solid image area becomes lower at an end portion thereof in a transporting direction of the latent-image-bearing member, the toner fails to transfer to the end portion of a halftone image area, and the image density is changed at the boundary between the solid image area and the halftone image area. In other words, abnormal images tend to appear at the boundary of image density, that is, the boundary between the adjacent latent images differing in electric potential, and at the point where the electric potentials of latent images suddenly show a discontinuous change. Such abnormal images are considered to result from transient development. Namely, only a toner component is transferred from the developer to the latent-image-bearing member while the developer passes the development nip. While a layer of developer that is a dielectric member with an electrostatic capacity supported by the developer-bearing member passes through a discontinuous electric field for development, abnormal images are easily produced. Such abnormal images caused by the discontinuous potential in latent images will be referred to as defective images in the present invention.
In recent years, in line with the trend toward a small-size developer unit, there has been a tendency for reduction in size of the developer-bearing member and the latent-image-bearing member. Therefore, the diameters of the currently available developer-bearing member and the latent-image-bearing member in a cylindrical form are both reduced, whereby the curvature radius is reduced at a position where the developer-bearing member is brought into the immediate proximity of the latent-image-bearing member. The result is that the development nip width (L) necessarily decreases. Therefore, curtailment of the nip time easily decreases the amount of developer to be brought into contact with the latent-image-bearing member in a manner similar to that as mentioned above. For preventing this problem from happening, it is proposed to make a difference between the aforementioned transporting velocities Vp and Vr larger. In this case, however, the defective images are also induced.
It is therefore a first object of the present invention to provide a developing method which does not produce defective images such as image blurring and intermission in thin line images, but can produce images with sufficient image density and excellent resolving power as a whole, particularly, in thin line images and small-size dot images.
A second object of the present invention is to provide a developer unit for use with the above-mentioned developing method.
The first object of the present invention can be achieved by a method for developing electrostatic latent images in such a configuration that an electrostatic-latent-image-bearing member is disposed to face a developer-bearing member which bears thereon a developer comprising a magnetic carrier and a toner, the method comprising the step of relatively moving a surface of the developer-bearing member and a surface of the electrostatic-latent-image-bearing member at different velocities to develop the electrostatic latent images with the toner, wherein the development is carried out under conditions represented by formula (1):                                           0.1            ⁢                          xe2x80x83                        ⁢            mm                    ≤          k                =                              L            ·                          [                                                (                                      Vr                    /                    Vp                                    )                                -                1                            ]                                ≤                      2            ⁢                          xe2x80x83                        ⁢            mm                                              (        1        )            
wherein Vp is a transporting velocity (mm/sec) of the surface of the electrostatic-latent-image-bearing member, Vr is a transporting velocity (mm/sec) of the surface of the developer-bearing member, and L is a width (mm) of a contact portion between the developer and the electrostatic-latent-image-bearing member.
The second object of the present invention can be achieved by a unit for developing electrostatic latent images comprising an electrostatic-latent-image-bearing member and a developer-bearing member which has a permanent magnet therein and bears thereon a developer comprising a magnetic carrier and a toner, wherein a surface of the developer-bearing member and a surface of the electrostatic-latent-image-bearing member are relatively moved at different velocities, and the electrostatic latent images are developed with the toner in such a configuration that the developer-bearing member is kept parallel to the electrostatic-latent-image-bearing member at a position where the developer-bearing member is located nearest to the electrostatic-latent-image-bearing member, with the development being carried out under conditions represented by formula (1):                                           0.1            ⁢                          xe2x80x83                        ⁢            mm                    ≤          k                =                              L            ·                          [                                                (                                      Vr                    /                    Vp                                    )                                -                1                            ]                                ≤                      2            ⁢                          xe2x80x83                        ⁢            mm                                              (        1        )            
wherein Vp is a transporting velocity (mm/sec) of the surface of the electrostatic-latent-image-bearing member, Vr is a transporting velocity (mm/sec) of the surface of the developer-beating member, and L is a width (mm) of a contact portion between the developer and the electrostatic-latent-image-bearing member.